1. Field of the Invention
The present invention relates to a magnetic recording medium having a large information recording capacity and a magnetic recording apparatus using such a magnetic recording medium. Particularly, the invention relates to a magnetic recording medium adapted for high-density magnetic recording, a method for producing such a magnetic recording medium, and a small-sized large-capacity magnetic recording apparatus using such a magnetic recording medium.
2. Description of Related Art
The demand for increase in the capacity of a magnetic recording apparatus has become higher and higher. An electromagnetic inductive magnetic head utilizing the voltage change caused by a flux change with the passage of time has been used as a related-art magnetic head. This head has an advantage that both recording and reproducing can be performed by one head. On the other hand, recently, a combination type head in which a recording head and a reproducing head are provided separately so that use of a more highly sensitive magneto-resistance effect type head for reproduction has been progressed rapidly. In order to improve the sensitivity of a magneto-resistance effect type head utilizing the change of electric resistance of a head device caused by the change of magnetic flux leaked from a medium, a still more highly sensitive head utilizing a very large change of magnetic resistance (a giant magneto-resistance effect or a spin valve effect) generated in magnetic layers of the type having a plurality of magnetic layers laminated through a non-magnetic layer has been put into practical use. This head utilizes a phenomenon in which the direction of relative magnetization of magnetic layers having the non-magnetic layer therebetween is changed in accordance with the magnetic field leaked from the medium to change magnetic resistance.
On the other hand, factors necessary for increasing the density of the magnetic recording medium are (1) establishing both surface flatness required by low floating of the magnetic head and avoidance of a head sticking phenomenon at the time of stopping the magnetic head, with greater improvement of durability against frictional motion, (2) low noise characteristics and (3) achievement of stable magnetic characteristics with a large process margin.
First, as a countermeasure to the head sticking phenomenon, Japanese Patent No. 2064981 (JP-B-4-62413) discloses a magnetic recording medium having an underlayer, a magnetic layer and a protective layer formed successively on a non-magnetic support, wherein: a landing zone is provided in the magnetic recording medium; a roughness forming layer for forming roughness on a surface of the protective film layer in the landing zone toward the magnetic layer is provided between the non-magnetic support and the underlayer in the landing zone; and the roughness forming layer has larger surface roughness than that of a main surface of the non-magnetic support and has a grain boundary of a film material. That is, the Japanese Patent No. 2064981 discloses a configuration in which a rough layer due to the grain boundary generated at the time of formation of a thin film of aluminum, or the like, is provided under the magnetic film to thereby give roughness to the surface of the protective film.
Further, JP-B-2547651 discloses a magnetic recording medium having a magnetic layer interposed between a protective layer and a surface of a non-magnetic support coated with a roughness-forming substance for forming roughness on the surface of the support, wherein: the roughness-forming substance is a single metal or an alloy having a melting point not higher than 1,100xc2x0 C.; a structure of a large number of islands discontinuous in the plane direction of the surface of the non-magnetic support is formed on the surface of the non-magnetic support by the roughness-forming substance; an underlayer for the magnetic layer is provided between the non-magnetic support and the magnetic layer; and an oxygen trap layer is provided between the non-magnetic support and the underlayer. That is, the above JP-B-2547651 discloses a configuration in which a roughness-forming layer having a discontinuous island structure of aluminum, or the like, is provided. The above JP-B-2547651 further describes that the oxygen trap layer prevents the rise of oxygen from the nonmagnetic support to thereby improve the crystallinity of the undercoat film and improve the magnetic characteristic on page 3, left column, lines 26-28. That is, the oxygen trap layer is provided between the non-magnetic support and the underlayer in order to improve the magnetic characteristic of the magnetic film.
As disclosed in PCT Patent Application PCT/JP96/02720 (WO 98/12698), the inventors of the present invention have found that it is easy and controllable, for forming a roughness-forming layer, to use a method of forming discontinuous island-like protrusions of an intermetallic compound by sputtering by use of a target of a peritectic alloy type composition having an intermetallic compound phase of aluminum and chrome, or the like.
Further, as described in JP-A-10-74314, it has been found that not only can the magnetic characteristic of the magnetic film be improved, but also medium noise, which is a problem particularly in a magneto-resistance effect type head, can be reduced if a non-magnetic alloy layer containing Co as a main component is provided under the underlayer. It has been further found that the non-magnetic alloy layer containing Co as a main component is excellent also in adhesion to a glass substrate which has been often used as a countermeasure to a shock on the magnetic disk device in recent years.
In order to improve the characteristic of the aforementioned magnetic recording medium, the inventors of the present invention have made experiments. As a result, it has been found that medium noise can be reduced more stably if a non-magnetic alloy layer containing Co as a main component is impregnated with Cr or Zr having a high tendency to be oxidized, and the surface of the non-magnetic alloy layer is exposed to an oxygen atmosphere to be slightly oxidized, as described in Japanese Patent Application 8-292451 (JP-A-10-143865). All the contents of this document are incorporated here by reference.
A magnetic disk was produced by the aforementioned technique as follows. A Coxe2x80x94Crxe2x80x94Zr alloy layer was provided as a first underlayer. A rough layer of an intermetallic compound was provided, as a second underlayer on the first underlayer, by sputtering a target of an Alxe2x80x94Cr composition. A Coxe2x80x94Crxe2x80x94Zr alloy layer was provided as a third underlayer on the second underlayer. After the surface of the third underlayer was slightly oxidized, a fourth undercoat film and a magnetic film were formed to produce a magnetic disk. As a result, it was however found that crashing (a phenomenon that the magnetic layer is worn out) occurred in a friction durability test under a severe condition that dust was introduced. The terminology xe2x80x9cfriction durability test under the condition that dust is introducedxe2x80x9d (hereinafter referred to as xe2x80x9cdust introduction testxe2x80x9d) means a test for measuring the number of seeking times to cause crashing when a seeking operation of the magnetic head is repeated after about 0.1 g of alumina particles each having a size of about 2 xcexcm is sprinkled onto the magnetic disk surface in the magnetic disk device.
To improve the aforementioned problem, a first object of the present invention is to provide a magnetic recording medium in which, while the low noise characteristic which is important for achieving high recording density is kept, high reliability of frictional durability can be secured even in the aforementioned severe friction durability test.
A second object of the present invention is to provide a method for producing such a magnetic recording medium.
To attain the foregoing objects, the inventors of the present invention have tested various materials for forming first, second and third underlayers. As a result, it has been found that a good result is obtained if an Nixe2x80x94Crxe2x80x94Zr alloy layer containing Ni as a substitute for Co is used as the third underlayer. It has been further found that a good result is obtained also in the case where a Coxe2x80x94Crxe2x80x94Zr alloy layer or an alloy containing Ni as a main component and also containing Cr and Zr is used as the first underlayer. The reason why a good result is obtained if an Nixe2x80x94Crxe2x80x94Zr alloy layer instead of a Coxe2x80x94Crxe2x80x94Zr alloy layer is used as the third underlayer is inferred as follows. This is because the Nixe2x80x94Crxe2x80x94Zr alloy layer is more highly amorphous than the Coxe2x80x94Crxe2x80x94Zr alloy layer so that the former is higher in toughness as a thin film than the latter. Further, when a fourth underlayer is formed on the third underlayer to serve as a film for controlling crystalline orientation, a magnetic layer having a good recording/reproducing characteristic in a high-density recording region can be formed on the fourth underlayer.
The present invention was attained as described above. According to an aspect of the present invention, a magnetic recording medium comprises a base; a first underlayer formed over said base; a second underlayer formed on said first underlayer and having a plurality of protrusions with a predetermined density; a third underlayer formed over said second underlayer and including an alloy containing Ni as a main component, at least one element selected from a first group consisting of Cr, Ti, V, Mo and Nb and at least one element selected from a second group consisting of Zr, Ta, Hf, Y and W; a fourth underlayer formed on said third underlayer; and a magnetic layer formed over said fourth underlayer.
According to another aspect of the present invention, a method for producing a magnetic recording medium comprises the steps of: (a) forming a first underlayer over a base by sputtering; (b) heating said base having said first underlayer after step (a); (c) forming a second underlayer on said first underlayer so that said second underlayer has a plurality of protrusions with a predetermined density; (d) forming a third underlayer over said second underlayer so that said third underlayer includes an alloy containing Ni as a main component, at least one element selected from a first group consisting of Cr. Ti, V, Mo and Nb, and at least one element selected from a second group consisting of Zr, Ta, Hf, Y and W; (e) heating said base formed with said first, second and third underlayers, after step (d); (f) forming a fourth underlayer on said third underlayer; and (g) forming a magnetic layer over said fourth underlayer.